Carbon, the sixth element of the Periodic Table, exists in various allotropic forms that span multiple dimensions, for example, 0D fullerenes, 1D nanotubes and 2D graphene. Each of these nano-allotropes has diverse properties and hence, varied applications. Graphene is a single-atom thick sheet of carbon atoms and that possesses desirable properties, such as, for example, high conductivity, high mechanical strength, high thermal conductivity, etc. Charge carrier concentration and mobility in graphene sheets are as high as 1013 cm−2 and 10,000 cm2 V−1s−1 at room temperature. Moreover, single layer graphene has a Young's modulus of ˜1 TPa, close to value of bulk graphite. Several synthesis procedures exist for large scale production of single and few layer graphene sheets. While the discovery of 2-D graphene by Novoselov and Geim lead to the 2010 physics Nobel prize, the zero band gap of 2-D graphene limits its electronic application, which is primarily based on semiconductors. The zero band gap of graphene can be overcome by modifying its size and shape, for example, reduction of the lateral size of the graphene sheets to small length scales (for example, a few nm), a band gap opens up making the material semiconducting.
Graphene nanoribbons address this drawback of single layer graphene, however, more recently, focus has been on another carbon nanostructure called graphene quantum dots (GQDs) or carbon quantum dots (CQD) (also known as graphene quantum discs). GQDs show very desirable photoluminescence properties, as the size and shape of the GQDs can be tuned to have desired band gap and emission properties. Moreover, GQDs have desirable characteristics, for example, high surface area, larger diameter, better surface grafting using the π-π conjugated network or surface groups and other special physical properties due to the structure of graphene. Since most of the carbon nanomaterials including GQDs are biocompatible and nontoxic, GQDs can advantageously be used in biological applications for example, image scanning and sensing, drug delivery and cancer treatment. The photoluminescence properties of GQDs are useful for photovoltaic applications too as it has been theoretically proved that the energy gap in GQDs can be tuned by using electrostatic potentials.
The band gap of a GQD depends on its size and shape. With existing technology it is possible to cut graphene in to desirable size and shape forms. As the number of atoms increases, the energy gap in almost all the energy spectra of GQDs decreases monotonously. In the case of GQDs, along with size and shape, the edge type plays an important role in electronic, magnetic and optical properties.
Typical synthesis procedures of GQDs include laser ablation treatment, solvothermal methods, hydrazine methods, hydrothermal methods, micro wave synthesis, chemical treatment of carbon fiber and bottom up methods. Depending on the synthesis procedure some GQDs are water soluble and some are not. Since the edges are highly active, functional groups may become attached, which may alter the solubility of the GQDs. The yield of production of GQDs in each of these typical procedures is less than 20%. In most of the cases a mixture of GQD and 1-D nanostructures and/or GQD and 2-D nanostructures is obtained.